Method for cracking butene

ABSTRACT

A method of producing propylene and ethylene from a butene-containing hydrocarbon stream by cracking olefin compounds in the butene-containing hydrocarbon stream in the presence of a core-shell ZSM catalyst, wherein the core-shell ZSM catalyst comprises a ZSM-5 core and a silica shell disposed thereon. Various embodiments of the method of producing propylene and ethylene, and the method of making the core-shell ZSM catalyst are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of Ser. No. 15/402,854, havinga filing date of Jan. 10, 2017, now allowed.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method of producing propylene andethylene from a butene-containing hydrocarbon stream by cracking olefincompounds present in the butene-containing hydrocarbon stream in thepresence of a surface modified core-shell ZSM catalyst.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Propylene and ethylene are important compounds in chemical andpetrochemical industries. They have been widely used to produce otheruseful compounds and polymers such as oxo alcohol, propylene oxide,cumene, methyl methacrylate, phenol, acrylic acid, isopropyl alcohol,acrylonitrile, oligomers, polyethylene, polypropylene, etc. A globalpropylene demand was estimated to be around 90 million tons in 2014,with an average annual growth rate of 4.6%.

The main sources of producing propylene in chemical and petrochemicalindustries are steam crackers or FCC units, wherein propylene isproduced as a byproduct of cracking heavier fractions of a hydrocarbonfeedstock. The amount of propylene that is coproduced in a steam crackergreatly depends on the composition of the hydrocarbon feedstock.However, with the recent decline in oil prices, cracking lighterhydrocarbons is no longer economically advantageous. The reduction ofcracking lighter hydrocarbons may result in a reduction of producingpropylene and ethylene. On the other hand, thermal cracking (or steamcracking), which is a large source of propylene production in refineriesand chemical industries, may form a large amount of methane (agreenhouse gas) as a by-product.

Alternatively, several propylene production processes are also availableas means for producing propylene. These processes includemethanol-to-olefins (MTO), propane dehydrogenation, catalytic crackingof butenes, and olefins metathesis. Among these, butenes cracking hasattracted attentions due to the availability of large and stablesupplies of butene from FCC and steam cracking processes. OlefinCracking Process (UOP-ATOFINA), Propylur (Lurgi), PCC process(Exxon-Mobil), SUPERFLEX® (Lyondell/Kellogg), and Mobil's Olefin Interconversion Process (MOI) are examples of commercially availableproduction processes for producing lower olefins such as ethylene andpropylene via hydrocarbon streams containing C₄-C₅.

Modified zeolite catalyst has been studied for cracking reactions ofbutene to propylene and ethylene. For example, Rongrong Zhang et al.(Chinese Journal of Chemical Engineering, 2015, v. 23, pp. 1131-1137)prepared a silver modified HZSM-5 zeolite catalyst via ion exchangemethod, and further used the catalyst towards catalytic cracking of1-butene. The maximum propylene yield was about 30% by mass and theactivity of the silver modified HZSM-5 zeolite catalyst was found to behigher when compared to the parent zeolite. Furthermore, Jianwen Li etal. (Fuel Processing Technology 2015, v. pp. 32-38) reported that aHZSM-5 zeolite catalyst, which has been modified by phosphorus and/oriron, enhanced selectivity of the cracking reactions towards propyleneformation. However, a gradual deactivation of the catalyst was observedin the phosphorus and/or iron modified catalyst. Accordingly, the buteneconversion was dropped by about 20%, in the first 25 hours of butenecracking. In another study, Higuchi et al. (U.S. Pat. No. 9,205,415 B2)disclosed a process for propylene production using MFI or MEL-typecatalyst having Si/Al atomic ratio of 500 to 1000. Effect of loadingdifferent alkali metal compound such as K, Na and Li were tested usingdimethyl ether as feed. A propylene yield of about 27% by mole wasobserved when 1-butene was used as a feed. Moreover, Midorikawa et al.(U.S. Pat. No. 9,192,922) disclosed propylene production using ZSM-5type catalyst modified with silica and phosphorous. The reaction wascarried out in fluidized bed reactor with a feed having an ethylene to1-butene weight ratio of 80:20. It was shown that after 3 hours of thereaction, the ethylene conversion was about 63.3% and the propyleneyield was about 21.2%. In addition, Van Westrenen et al. (U.S. Pat. No.8,822,749B2) disclosed mixed catalyst system having MFI (ZSM-5) and TON(ZSM-22) or MFI (ZSM-5) and MTT (ZSM-23) type molecular sieves forpropylene production using dimethyl ether and 1-butene as a feed. Mixedcatalyst system slightly enhances the propylene production when comparedto single catalyst system. Besides, Al-Khattaf et al. (US PatentApplication No. 2016/0130197 A1) disclosed a process for cracking C₄olefins to propylene and ethylene via an MFI zeolite catalyst having aSi/Al molar ratio of 2000. It was shown that acid-treating orbase-treating the catalyst increases the propylene production yield. Theincreased propylene yield after base-treating the catalyst may be due toan enrichment of silanol group in the catalyst. Knowing the abovedisclosure, however, there appear to be no report on utilizing a surfacemodified ZSM-5 as a cracking catalyst for the production of lightolefins such as ethylene and propylene from a feed containing butene.

In view of the forgoing, one objective of the present invention is toprovide a method of producing propylene and ethylene from abutene-containing hydrocarbon stream by cracking olefin compounds in thebutene-containing hydrocarbon stream in the presence of a surfacemodified core-shell ZSM catalyst. Further embodiments of the presentinvention relates to methods of making the core-shell ZSM catalyst.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect the present disclosure relates to a methodof producing propylene and ethylene from a butene-containing hydrocarbonstream, involving contacting the butene-containing hydrocarbon streamwith a core-shell ZSM catalyst to form a product stream comprisingpropylene and ethylene, wherein the core-shell ZSM catalyst includes aZSM-5 core, and a silica shell having a thickness in the range of 0.5 to50 m, which covers at least a portion of a surface of the ZSM-5 core.

In one embodiment, the core-shell ZSM catalyst is dispersed in a silicaand/or an alumina binder.

In one embodiment, a weight percent of the silica shell in thecore-shell ZSM catalyst is within the range of 4 to 75 wt %, with theweight percent being relative to the total weight of the core-shell ZSMcatalyst.

In one embodiment, the core-shell ZSM catalyst has an acidity of lessthan 0.1 mmol/g.

In one embodiment, at least 50 wt % of the product stream is propyleneand ethylene.

In one embodiment, a propylene-to-ethylene weight ratio of the productstream is within the range of 0.2 to 4.

In one embodiment, the method of producing propylene and ethylene fromthe butene-containing hydrocarbon stream further involves treating thecore-shell ZSM catalyst with nitrogen at a temperature in the range of400 to 700° C. prior to the contacting.

In one embodiment, the method of producing propylene and ethylene fromthe butene-containing hydrocarbon stream further involves mixing thebutene-containing hydrocarbon stream with nitrogen to form a gaseousmixture prior to the contacting, wherein a partial pressure of thebutene-containing hydrocarbon stream in the gaseous mixture is withinthe range of 5 to 50 psi.

In one embodiment, the butene-containing hydrocarbon stream is contactedwith the core-shell ZSM catalyst a temperature in the range of 400 to700° C., and a space velocity in the range of 800 to 10,000 h⁻¹.

According to a second aspect the present disclosure relates to a methodof making a core-shell ZSM catalyst, involving i) treating a ZSM-5zeolite with a silicalite gel including a silicating agent, a structuredirecting agent, a mineralizing agent, and water at a temperature in therange of 150° C. to 250° C. and a pressure in the range of 2 to 20 bars,ii) calcining the core-shell ZSM catalyst to form the core-shell ZSMcatalyst having a silica shell with a thickness of 0.5 to 15 μm, iii)treating the core-shell ZSM catalyst with the silicalite gel, iv)repeating the calcining and the treating until the thickness of thesilica shell is in the range of 0.5 to 50 μm.

In one embodiment, the core-shell ZSM catalyst is treated once but nomore than three times.

In one embodiment, a weight percent of the silica shell in thecore-shell ZSM catalyst is within the range of 4 to 75 wt %, with theweight percent being relative to the total weight of the core-shell ZSMcatalyst.

In one embodiment, a ratio of an acidity of the core-shell ZSM catalystto the ZSM-5 zeolite is no more than 0.2.

In one embodiment, the structure directing agent is a quaternaryammonium salt with a chemical formula N(C_(n)H_(3n))₄ ⁺X⁻, with X beinga halide ion or a hydroxide, and n being an integer between 1 to 5.

In one embodiment, the mineralizing agent is a fluoride salt.

In one embodiment, the silicating agent is silica.

According to a third aspect the present disclosure relates to a methodof making a core-shell ZSM catalyst, involving i) treating a ZSM-5zeolite with a silicating solution comprising an organic solvent and asilicating agent at a boiling point of the organic solvent, ii)calcining the core-shell ZSM catalyst to form the core-shell ZSMcatalyst having a silica shell with a thickness of 0.5 to 5 μm, iii)treating the core-shell ZSM catalyst with the silicating solution, iv)repeating the calcining and the treating until the thickness of thesilica shell is in the range of 0.5 to 30 μm.

In one embodiment, the core-shell ZSM catalyst is treated once but nomore than six times.

In one embodiment, a ratio of an acidity of the core-shell ZSM catalystto the ZSM-5 zeolite is in the range of 0.45 to 0.65.

In one embodiment, the silicating agent is tetraethylorthosilicateand/or aminopropyltrimethoxysilane.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A illustrates a catalyst bed in the form of a matrix comprising acore-shell ZSM catalyst and a binder.

FIG. 1B is a magnified illustration of the core-shell ZSM catalysthaving a ZSM-5 core and a silica shell.

FIG. 1C is a magnified illustration of the silica shell.

FIG. 1D is a magnified illustration of the ZSM-5 core.

FIG. 2A is a SEM micrograph of a surface of the ZSM-5 core.

FIG. 2B is a SEM micrograph of a surface of the core-shell ZSM catalyst,which is made by treating the ZSM-5 core with a silicating solution sixtimes.

FIG. 2C is a SEM micrograph of a surface of the core-shell ZSM catalyst,which is made by treating the ZSM-5 core with a silicalite gel, whereinzeolite crystals have a size of about 2 μm.

FIG. 2D is a SEM micrograph of a surface of the core-shell ZSM catalyst,which is made by treating the ZSM-5 core with a silicalite gel twotimes, wherein zeolite crystals have a size of about 3.5 μm.

FIG. 3 represents a yield of cracking C₄₋₁₂ olefin compounds (e.g.butene) to ethylene and propylene via different catalysts.

FIG. 4 represents time-on-stream behavior of cracking butene topropylene and ethylene over the core-shell ZSM catalyst (catalyst C4),and the ZSM-5 core (catalyst D).

DETAILED DESCRIPTION OF THE EMBODIMENTS

According to a first aspect the present disclosure relates to a methodof producing propylene and ethylene from a butene-containing hydrocarbonstream, involving contacting the butene-containing hydrocarbon streamwith a core-shell ZSM catalyst to form a product stream comprisingpropylene and ethylene.

The term “butene-containing hydrocarbon stream” refers to a hydrocarbonstream that contains butene and may further include C₅₋₁₂ olefincompounds. The term “butene”, as used here, refers to all isomers ofbutene, including 1-butene, cis-2-butene, trans-2-butene, and/orisobutylene (also known as 2-methylpropene). Preferably, at least 50 wt%, more preferably at least 60 wt %, even more preferably at least 70 wt% of the butene-containing hydrocarbon stream is butene, with the weightpercent being relative to the total weight of the butene-containinghydrocarbon stream.

The term “olefin” as used here refers to unsaturated straight-chainhydrocarbons, unsaturated branched hydrocarbons, or cyclic olefins. Inone embodiment, the butene-containing hydrocarbon stream furtherincludes at least one raw material selected from the group consisting ofC₁₋₁₂ hydrocarbons, such as for example C₄₋₁₂ normal paraffins,isoparaffins, cycloparaffins (i.e. naphthenes), cycloparaffins havingside chain alkyl groups, aromatics, and aromatics having side chainalkyl groups. For example, in one embodiment, the butene-containinghydrocarbon stream includes ethylene, propylene, 1-butene, cis- andtrans-2-butene, pentene, hexene, methane, ethane, propane, butane,pentane, hexane, benzene, toluene, xylenes, and ethylbenzene. Thebutene-containing hydrocarbon stream may include at least 60 wt %,preferably at least 70 wt %, more preferably at least 80 wt % of C₄₋₁₂olefin compounds, and preferably no less than 30 wt % of C₄₋₁₂ olefincompounds. A production yield of converting olefin to propylene andethylene may not be considerable for a butene-containing hydrocarbonstream having an olefin content of less than 30 wt %.

In one embodiment, the butene-containing hydrocarbon stream may alsoinclude small quantities (i e less than 2 wt %, preferably less than 1wt %) of oxygen-containing compounds (e.g. tert-butanol, methyltert-butyl ether, methanol, etc). Further, the butene-containinghydrocarbon stream may include diolefin (diene) compounds e.g.propadiene, butadiene, and/or pentadiene. However, the diolefincompounds are highly polymerizable and may poison the core-shell ZSMcatalyst, therefore a content of diolefin compounds may preferably beless than 1 wt %, preferably less than 0.5 wt %. In circumstances wherethe content of diolefin compounds is above 2 wt %, preferably above 5 wt%, the butene-containing hydrocarbon stream may be treated in adistillation or a partial hydrogenation unit to reduce the diolefincontent down to less than 1 wt %, preferably less than 0.5 wt %.

In some embodiments, the butene-containing hydrocarbon stream may be aC₄₋₅ fraction of naphtha, which is isolated from products of thermalcracking of naphtha. The C₄₋₅ fraction may partially be hydrogenated topartially convert diolefins into olefins. The butene-containinghydrocarbon stream may be a C₄ fraction and a gasoline fraction isolatedfrom products of fluidized catalytic cracking (FCC) of vacuum gas oiland other petroleum hydrocarbons. Alternatively, the butene-containinghydrocarbon stream may be a C₄ fraction and a gasoline fraction isolatedfrom cokers. The butene-containing hydrocarbon stream may be a C₄fraction and a gasoline fraction isolated from hydrocarbons synthesizedby Fischer-Tropsch reaction (FT synthesis) from carbon monoxide andhydrogen. The butene-containing hydrocarbon stream obtained from theseaforementioned sources may be used individually, or two or more may beused as a mixture. Alternatively, the butene-containing hydrocarbonstream may be an effluent of an ethylene cracker, a steam cracker, aseparation column, or a combination thereof.

According to the method, the butene-containing hydrocarbon stream isbrought into contact with the core-shell ZSM catalyst in a reactor toconvert at least a portion of C₄₋₁₂ olefin compounds present in thebutene-containing hydrocarbon stream to propylene and/or ethylene via acatalytic dealkylation reaction.

Contacting as used herein refers to a process whereby a liquid-stateand/or a vaporized-state of a hydrocarbon-containing stream is directlycontacted with a catalyst present in a catalyst bed of a reactor atreaction conditions that are favored for catalytic reactions to takeplace. Accordingly, in some embodiments, the butene-containinghydrocarbon stream is contacted with the core-shell ZSM catalyst bypassing through the catalyst when in a liquid state, or by passingthrough and/or over the catalyst when in a vaporized-state (e.g. as anatmosphere to the catalyst). Additionally, the butene-containinghydrocarbon stream may be mixed with the core-shell ZSM catalyst to forma heterogeneous mixture. Among these, the preferred contacting is bypassing the butene-containing hydrocarbon stream through the core-shellZSM catalyst.

The reactor may be a cylindrical vessel that is vertically orhorizontally oriented. Preferably the reactor is a vertically orientedcylindrical vessel, wherein the butene-containing hydrocarbon streamenters the reactor from a top end when the feed is in a liquid state,and the butene-containing hydrocarbon stream enters the reactor from abottom end when the feed is in a vaporized state (e.g. the gaseousmixture). The reactor may be a fixed-bed reactor, a moving-bed reactor,or a fluidized-bed reactor, wherein the core-shell ZSM catalyst isbrought into contact with the butene-containing hydrocarbon stream.Preferably, the reactor is a fixed-bed reactor. In one embodiment, thereactor is filled with the core-shell ZSM catalyst, and thebutene-containing hydrocarbon stream is contacted with the core-shellZSM catalyst at suitable reaction conditions (e.g. at a temperature inthe range of 400 to 700° C., more preferably 500 to 650° C., and atatmospheric pressure or a pressure in the range of 0.01 to 1 MPa,preferably in the range of 0.05 to 0.3 MPa), whereby at least a portionof the C₄₋₁₂ olefin compounds is converted to propylene and/or ethylene.

The catalyst bed may be a container with an internal cavity that isfilled with the core-shell ZSM catalyst, and is disposed inside thereactor. Examples of the catalyst beds include, but are not limited to ahollow tube, a pipe, a duct, etc. In a preferred embodiment, the reactoris a cylindrical vessel with the catalyst bed located inside, whereinthe butene-containing hydrocarbon stream is contacted with thecore-shell ZSM catalyst present in the catalyst bed. The reactor mayinclude one or more catalyst beds that are located in series, however,preferably, the reactor has only one catalyst bed. The reactor may havevarious geometries including spherical, conical, pyramidal, rectangular,or cubical geometries. In one embodiment, the reactor has a volume inthe range of 0.01-10,000 L, preferably 1-1,000 L, more preferably100-1,000 L. Further to the core-shell ZSM catalyst, inert materials maybe present in the reactor bed, for example to moderate hot spotsthroughout the reactor bed. Preferably the inert material is clay, sand,and/or gravel. The inert material may also be silica and/or alumina. Theinert materials may be in the form of balls or pellets to catchimpurities and to assist in flow distribution of the reactants throughthe catalyst.

A catalytic dealkylation reaction (or dealkylation) refers to a chemicalreaction whereby one or more alkyl groups (e.g. methyl, ethyl, propyl,butyl, etc.) are removed from a hydrocarbon compound, for example,dealkylation of methylethylbenzene to form toluene. Dealkylationgenerally takes place in the presence of a catalyst. Further to thedealkylation reactions, other unfavorable catalytic reactions such astransalkylation or disproportionation may take place in the reactor.Transalkylation refers to a chemical reaction through which one or morealkyl groups (e.g. methyl, ethyl, propyl, butyl, etc.) are transferredfrom one organic compound to another. For example, transalkylation of amixture containing toluene and trimethylbenzene may lead to theformation of xylene. Furthermore, disproportionation refers to a redoxreaction in which an organic molecule is reduced in a reductionreaction, and the similar organic molecule is also oxidized in anoxidation reaction, thereby forming two different products. For example,disproportionation of toluene may form benzene (via an oxidationreaction), and xylene (via a reduction reaction).

In one embodiment, the method of producing propylene and ethylene fromthe butene-containing hydrocarbon stream further involves treating thecore-shell ZSM catalyst with nitrogen prior to the contacting.Accordingly, the core-shell ZSM catalyst is first heated to atemperature in the range of 400 to 700° C., preferably 450-650° C., morepreferably about 550-600° C., preferably in a sub-atmospheric pressure.Thermal treatment of the core-shell ZSM catalyst at a sub-atmosphericpressure may improve the resistance of the core-shell ZSM catalystagainst coking deterioration. Then nitrogen is purged over thecore-shell ZSM catalyst an atmospheric pressure, preferably at apressure in the range of 2-10 atm. In one embodiment, the core-shell ZSMcatalyst is held isothermally for at least 6 hours, preferably at least8 hours, more preferably at least 10 hours, while purging with nitrogen.

In a preferred embodiment, after the core-shell ZSM catalyst is treatedwith nitrogen, the butene-containing hydrocarbon stream is mixed with adilution gas stream to form a gaseous mixture prior to contacting thebutene-containing hydrocarbon stream with the core-shell ZSM catalyst.In this embodiment, the gaseous mixture is passed through and/or overthe core-shell ZSM catalyst to increase a contact surface area betweenthe gaseous mixture and the core-shell ZSM catalyst. Preferably, apartial pressure of the butene-containing hydrocarbon stream in thegaseous mixture is within the range of 5 to 50 psi, more preferably10-50 psi, even more preferably 20-40 psi. In one preferred embodiment,the butene-containing hydrocarbon stream is mixed with the dilution gasstream in an upstream mixer, and prior to being delivered to thereactor. However, in one embodiment, the butene-containing hydrocarbonstream is mixed with the dilution gas stream inside the reactor. Thedilution gas stream may be a nitrogen stream, a carbon dioxide stream, ahelium stream, and/or a methane stream.

Preferably, the butene-containing hydrocarbon stream is contacted withthe core-shell ZSM catalyst a temperature in the range of 400 to 700°C., more preferably 500 to 650° C., even more preferably 540 to 600° C.In addition, in another embodiment, the butene-containing hydrocarbonstream (or the gaseous mixture) is contacted with the core-shell ZSMcatalyst at an atmospheric pressure or a pressure in the range of 0.01to 1 MPa, preferably in the range of 0.05 to 0.3 MPa, more preferably0.05 to 0.2.

Furthermore, the butene-containing hydrocarbon stream is contacted withthe core-shell ZSM catalyst at a space velocity in the range of 800 to10,000 h⁻¹, preferably 1,000 to 5,000 h⁻¹, more preferably 1,000 to2,000 h⁻¹. The term “space velocity” refers to a ratio of the volumetricflow rate of the influent of a reactor to a volume of the reactor (orthe catalyst bed volume). Space velocity indicates how many reactorvolumes of feed (e.g. the butene-containing hydrocarbon stream) can betreated per unit time. For example, a reactor with a space velocity of 5h⁻¹ is capable of processing a feed with a volume that is equivalent tofive times the reactor volume in each hour. A contact time between thebutene-containing hydrocarbon stream and the core-shell ZSM catalyst ispreferably 30 seconds or less, more preferably 10 second or less.

The product stream, which egresses the reactor in a liquid form and/orin a vapor form, includes propylene, ethylene, and one or more of C₄₋₁₂olefin/diolefin compounds (e.g. cis- and trans-butene, pentene, hexene,propadiene, butadiene, pentadiene, cyclopentadiene, dicyclopentadiene,etc.), C₁₋₁₂ hydrocarbons such as n-paraffins, i-paraffins,cycloparaffins, and aromatics (e.g. methane, ethane, propane, butane,pentane, cyclopentane, hexane, cyclohexane, benzene, toluene, xylenes,ethylbenzene, etc.), oxygen-containing compounds (e.g. tert-butanol,methyl tert-butyl ether, methanol, etc), nitrogen, hydrogen, watervapor, and carbon dioxide.

In a preferred embodiment, at least 50 wt %, more preferably at least 55wt %, even more preferably at least 60 wt % of the product stream ispropylene and ethylene, whereas less than 20 wt %, preferably less than15 wt %, more preferably less than 10 wt % of the product streamincludes C₁₋₁₂ paraffin compounds such as n-paraffins, i-paraffins, andcycloparaffins (e.g. methane, ethane, propane, butane, pentane,cyclopentane, hexane, cyclohexane). Accordingly, the product stream ofthis embodiment may also include less than 40 wt %/o, more preferablyless than 35 wt %, even more preferably less than 30 wt % of C₄1₂olefin/diolefin compounds (e.g. cis- and trans-butene, pentene, hexene,propadiene, butadiene, pentadiene, cyclopentadiene, dicyclopentadiene,etc.), with the weight percent being measured relative to the totalweight of the product stream. In another preferred embodiment, apropylene-to-ethylene weight ratio (or P/E ratio) of the product streamis within the range of 0.2 to 4, preferably 1.0 to 3.5, more preferablyabout 3.0.

In a preferred embodiment, the yield of formation of propylene andethylene is in the range of 40-65%, preferably 45-60%, more preferably45-55%, with the yield being to measured as the total weight ofpropylene and ethylene produced relative to the total weight of theC₄₋₁₂ olefin compounds present in the butene-containing hydrocarbonstream. In a preferred embodiment, the product stream of this embodimentincludes less than 35 wt %, more preferably less than 30 wt % of C₄₋₁₂olefin compounds (e.g. cis- and trans-butene, pentene, hexene, heptene,etc.), and a portion of the product stream is recycled back to thereactor and reacted again to increase the yield of formation ofpropylene and ethylene. According to this embodiment, the yield offormation of propylene and ethylene may increase to a range of 50-80%,preferably 60-70%, with the yield being measured as the total weight ofpropylene and ethylene produced relative to the total weight of theC₄₋₁₂ olefin compounds initially present in the butene-containinghydrocarbon stream.

In a preferred embodiment, the yield of formation of propylene andethylene stays within the range of 40-65%, preferably 45-60%, morepreferably 45-55%, over a period of at least 50 hours, preferably atleast 100 hours, more preferably at least 200 hours. However, in anotherembodiment, the core-shell ZSM catalyst may be deactivated via cokingdeterioration in the reactor, when the catalytic reactions are carriedout for more than 200 hours. According to this embodiment, thecore-shell ZSM catalyst can be regenerated by burning off the coke onthe catalyst at a temperature of 400 to 700° C., in an atmosphere of airor a gaseous mixture of oxygen and an inert gas.

In one embodiment, the method in accordance with the first aspectfurther involves separating propylene and/or ethylene from the productstream. Accordingly, a concentration of the C₁₋₁₂ hydrocarbons such asn-paraffins, i-paraffins, and cycloparaffins (e.g. methane, ethane,propane, butane, pentane, cyclopentane, hexane, cyclohexane, etc.) maybe reduced to less than 2 wt %, preferably less than 1 wt %, morepreferably less than 0.5 wt % via a stripping column. Stripping refersto a process whereby one or more components of a liquid stream (e.g.C₁₋₁₂ paraffin hydrocarbons) are removed by a vapor stream. Accordingly,a stripping column is a vessel, wherein a liquid phase is in directcontact with a vapor phase at a condition favorable for one or morecomponents of the liquid phase to transfer to the vapor phase via a masstransport phenomenon. Exemplary stripping columns used herein mayinclude, but are not limited to tray towers, packed columns, spraytowers, and bubble columns. The product stream may further be processedto remove other substances present in the product stream, for example,C₄₋₁₂ olefin/diolefin compounds (e.g. cis- and trans-butene, pentene,hexene, propadiene, butadiene, pentadiene, cyclopentadiene,dicyclopentadiene, etc.), oxygen-containing compounds (e.g.tert-butanol, methyl tert-butyl ether, methanol, etc), nitrogen,hydrogen, water vapor, carbon dioxide, etc.

Referring now to FIGS. 1A, 1B, 1C, and 1D. The core-shell ZSM catalyst100 includes a ZSM-5 core 112 and a silica shell 110 that covers atleast a portion of the ZSM-5 core.

Zeolites are porous aluminosilicate minerals that occur in nature.Elementary building units of zeolites are SiO₄ and AlO₄ tetrahedra.Adjacent tetrahedra are linked at their corners via a common oxygenatom, which results in an inorganic macromolecule with athree-dimensional framework. The three-dimensional framework of azeolite also comprises channels, channel intersections, and/or cageshaving dimensions in the range of 0.1-10 nm, preferably 0.2-5 nm, morepreferably 0.2-2 nm. Water molecules may be present inside thesechannels, channel intersections, and/or cages.

In one embodiment, the core-shell ZSM catalyst 100 has a zeolite core,which is at least one zeolite selected from the group consisting of a4-membered ring zeolite, a 6-membered ring zeolite, a 10-membered ringzeolite, and a 12-membered ring zeolite. The zeolite catalyst may have azeolite with a natrolite framework (e.g. gonnardite, natrolite,mesolite, paranatrolite, scolecite, and tetranatrolite), edingtoniteframework (e.g. edingtonite and kalborsite), thomsonite framework,analcime framework (e.g. analcime, leucite, pollucite, and wairakite),phillipsite framework (e.g. harmotome), gismondine framework (e.g.amicite, gismondine, garronite, and gobbinsite), chabazite framework(e.g. chabazite-series, herschelite, willhendersonite, and SSZ-13),faujasite framework (e.g. faujasite-series, Linde type X, and Linde typeY), mordenite framework (e.g. maricopaite and mordenite), heulanditeframework (e.g. clinoptilolite and heulandite-series), stilbiteframework (e.g. barrerite, stellerite, and stilbite-series), brewsteriteframework, or cowlesite framework. In one embodiment, the zeolite coreis a zeolite selected from the group consisting of ZSM-5, ZSM-8, ZSM-11,ZSM-12, ZSM-18, ZSM-23, ZSM-35 and ZSM-39. Of these zeolites, the mostdesirable types of zeolites are those represented as MFI structuresaccording to the IUPAC nomenclature for zeolite frameworks. For example,in a preferred embodiment, the core-shell ZSM catalyst 100 has the ZSM-5core 112. ZSM-5 is an aluminosilicate zeolite belonging to the pentasilfamily of zeolites that has an MFI-type framework with a 10-memberedring structure. ZSM-5 is widely used in chemical, petro-chemical, andpetroleum industries, for example as a heterogeneous catalyst forisomerization reaction of hydrocarbon compounds. Preferably, the ZSM-5core 112 has a SiO₂/Al₂O₃ molar ratio in the range of 23-10,000, orpreferably 23-2,000, or preferably 80-1,500, or preferably 200-400, orpreferably about 300. To measure the SiO₂/Al₂O₃ molar ratio of the ZSM-5core, the ZSM-5 core is dissolved in a strong aqueous alkali solution ora hydrofluoric acid solution, and the resulting solution is analyzed byplasma emission spectrometry, or the like to determine the SiO₂/Al₂O₃molar ratio. The term “core” in the “ZSM-5 core” refers to the “zeolitecore” in the “core-shell ZSM catalyst”.

The ZSM-5 core 112 includes micro-pores (i.e. pores with an average porediameter of less than 2 nm) having a specific pore volume in the rangeof 0.1-0.3 cm³/g, preferably 0.1-0.2 cm³/g, more preferably 0.15-0.2cm³/g. Preferably, an average pore diameter of the ZSM-5 core 112 iswithin the range of 4-12 Å, preferably 5-8 Å, more preferably 5-6.5 Å.The ZSM-5 core 112 may further include meso-pores (i.e. pores with anaverage pore diameters in the range of 2-50 nm, preferably 2-20 nm)having a specific pore volume in the range of 0.01-0.15 cm³/g,preferably 0.05-0.15 cm³/g, more preferably 0.05-0.1 cm³/g. In oneembodiment, a specific surface area of the micro-pores in the ZSM-5 core112 is in the range of 100-500 m²/g, preferably 300-500 m²/g, morepreferably about 400 m²/g, whereas a specific surface area of themeso-pores in the ZSM-5 core 112 is in the range of 50-150 m²/g,preferably 50-100 m²/g, more preferably about 80 m²/g. The core-shellZSM catalyst 100 may be in the form of pellets having a diameter in therange of 0.5-5 mm, preferably 0.5-1.5 mm, more preferably about 1 mm.The core-shell ZSM catalyst 100 may also be extrudated to have ageometry selected from the group consisting of cylindrical, rectilinear,star-shaped, conical, pyramidal, rectangular, cubical, and ring-shaped.

In one embodiment, an acidity of the ZSM-5 core 112 is in the range of0.01-2 mmol/g, preferably 0.1-1.5 mmol/g, more preferably 0.5-1.5mmol/g. Alternatively, in another preferred embodiment, the ZSM-5 coreincludes substantially no protons. The term “containing substantially noprotons” means that the amount of protons (i.e. the acidity) of theZSM-5 core is 0.7 mmol or less, per each gram of zeolite. Preferably,the amount of protons is 0.01 mmol or less per gram of zeolite.

Zeolite catalysts having weak acid sites may be effective in catalyzingthe formations of ethylene and propylene. On the other hand, zeolitecatalysts having strong acidic sites may catalyze the formations ofalkanes and aromatics. Forming a silica shell on the ZSM-core, as in thecore-shell ZSM catalyst, protonates light olefin compounds to formcarbocations thereby initiating acid-catalyzed reactions. The absence ofstrong acid sites in the core-shell ZSM catalyst may inhibit formedolefins (e.g. propylene or ethylene) to convert to alkanes or aromaticsvia hydrogen transfer reactions.

In one embodiment, a liquid phase ion exchange/filtrate titration methodis used to determine an acidity of a zeolite. Accordingly, the acidityof the ZSM-5 core is measured as follows. First, the ZSM-5 core iscalcined in air and then subjected to an ion exchange treatment in thepresence of an aqueous sodium chloride solution. After the treatment,the solution is filtered to obtain a filtrate. The filtrate is washedwith pure/deionized water, and the whole amount of the washing liquid iscollected and further mixed with the aforementioned filtrate. Theacidity of the resulting mixed solution (i.e. filtrate+washing liquid)is measured by neutralization titration, and a value per grams of thezeolite (i.e. the ZSM-5 core) is reported as the acidity of the zeolite(i.e. the ZSM-5 core). Alternatively, in a preferred embodiment, atemperature-programmed desorption (TPD) method is used to measure anacidity of a zeolite. Accordingly, a predetermined amount of the zeoliteis pretreated at 500° C. in the presence of helium for at least 60minutes, preferably 90 minutes. Then, the zeolite is treated at 100° C.in an atmosphere containing helium and 5 to 15 vol %, preferably 5 to 10vol % of ammonia. After that, the zeolite is heated at a temperatureramp rate of 10° C.min⁻¹ from 100° C. to 600° C., wherein ammonia isdesorbed, and the amount of desorbed ammonia (measured by athermogravimetric analyzer) is used to determine the acidity of thezeolite.

In one embodiment, the ZSM-5 core 112 includes at least one transitionmetal selected from the groups 6-12 of the period table, such as V, Cr,Mo, W, Mn, Pt, Pd, Fe, Ni, Zn, Ga, and Re. Preferably, the transitionmetal is selected from the group 11 of the periodic table. Accordingly,the transition metal may be at least one selected from the groupconsisting of copper, silver, and gold. Of these metals, copper andsilver are more preferred, wherein silver is the most preferred. In analternative embodiment, the ZSM-5 core 112 may contain the transitionmetals in an oxide form. The transition metals may be incorporated intothe ZSM-5 core via ion exchange, impregnation, and/or kneading, althoughthe preferable method is ion-exchange. In order to incorporate atransition metal into the ZSM-5 core, the ZSM-5 core is treated in ametal salt, which may preferably be selected from the group consistingof silver nitrate, silver acetate, silver sulfate, copper chloride,copper sulfate, copper nitrate, and gold chloride. An amount of thetransition metal in the ZSM-5 core may be in the range of 0.1 wt % to 5wt %, preferably 0.5 wt % to 3 wt %, with the weight percent beingrelative to the total weight of the ZSM-5 core. In a preferredembodiment, the transition metals are evenly dispersed within the ZSM-5core. The ZSM-5 core includes intra-crystalline and may also includeinter-crystalline pores, and the transition metals may be located at theinter-crystalline pores or preferably at the intra-crystalline pores. Inanother embodiment, some or all of the aluminum atoms in the frameworkstructure of the ZSM-5 core are replaced by atoms of Ga, Fe, B, and/orCr.

Although, the silica shell 110 preferably covers a surface of the ZSM-5core 112 entirely, in some alternative embodiments, the silica shell maycover at least 50%, or at least 80% of the surface of the ZSM-5 core.Preferably, the silica shell 110 has a thickness in the range of 0.5 to50 μm, or preferably 5-40 μm, or preferably 10-30 μm, or preferably15-30 μm.

A weight percent of the silica shell 110 in the core-shell ZSM catalyst100 may depend on how the silica shell is formed on the ZSM-5 core,since the method of forming the silica shell on the core affects thethickness of the shell. For example, in one embodiment, the silica shell110 is deposited on the ZSM-5 core 112 via a chemical liquid depositionmethod, wherein a weight percent of the silica shell in the core-shellZSM catalyst is within the range of 4 wt % to 30 wt %, preferably 15 wt% to 30 wt %, even more preferably 15 wt % to 25 wt %, with the weightpercent being relative to the total weight of the core-shell ZSMcatalyst. Alternatively, in another embodiment, the silica shell 110 isdeposited on the ZSM-5 core 112 via a hydrothermal method, wherein aweight percent of the silica shell in the core-shell ZSM catalyst iswithin the range of 4 to 75 wt %, preferably 30 to 75%, more preferably50 to 75 wt %, with the weight percent being relative to the totalweight of the core-shell ZSM catalyst. Each method of forming the silicashell on the ZSM-5 core is discussed in detail in the second and thirdaspect of the present disclosure.

The silica shell is configured to adjust the acidity of the core-shellZSM catalyst 100 to be within a predetermined acidity by altering athickness of the silica shell 110. The predetermined acidity may dependon how the silica shell is formed on the ZSM-5 core, since the method offorming the silica shell on the core affects the thickness of the shell.The method of forming the silica shell on the core may also affect thesize of zeolite crystals of the ZSM-5 core. For example, in oneembodiment, the silica shell 110 is deposited on the ZSM-5 core 112 viaa chemical liquid deposition method, whereby a core-shell ZSM catalystwith a predetermined acidity in the range of 0.1-1 mmol/g, preferably0.2-0.75 mmol/g is formed. In an alternative embodiment, the silicashell 110 is deposited on the ZSM-5 core 112 via a hydrothermal method,whereby a core-shell ZSM catalyst with a predetermined acidity of lessthan 0.1 mmol/g, preferably less than 0.05 mmol/g is formed. Each methodof forming the silica shell on the ZSM-5 core is discussed in detail inthe second and third aspect of the present disclosure.

As used herein, the term “silica shell” refers to a distinct silicacoating/layer of the core-shell ZSM catalyst with a measurablethickness. In the embodiments where the catalyst is admixed with abinder (e.g. a silica binder) in the catalyst bed, the binder issubstantially different than the silica shell (i.e. the binder such as asilica binder is entirely different from the silica shell, and is notconsidered as part of the core-shell ZSM catalyst).

Preferably, the propylene-to-ethylene weight ratio (or P/E ratio) of theproduct stream can be adjusted by varying the silica shell 110 thickness(and thus an acidity) of the core-shell ZSM catalyst 100. Furthermore,the P/E ratio of the product stream can be altered by altering the sizeof zeolite crystals of the ZSM-5 core. The P/E ratio adjustment of theproduct stream may be determined with respect to market or customerneeds.

Referring now to FIG. 1A. In one embodiment, the core-shell ZSM catalyst100 is dispersed in a binder 104 to form a matrix 102 including thebinder 104 and the core-shell ZSM catalyst 100 dispersed therein.Accordingly, a porous, flame-resistant, inorganic oxide such as alumina,silica, zirconia, titania, diatomaceous earth, and/or clay may be mixedas the binder 104 with the core-shell ZSM catalyst 100 to obtain acatalyst mixture that can be molded to form the matrix 102. The matrix102 can further be used as a catalyst bed in a reactor. According tothis embodiment, the core-shell ZSM catalyst 100 is present in thematrix 102 with a weight percent in the range of 10 wt % to 90 wt %,preferably 20 wt % to 50 wt %, with the weight percent being relative tothe total weight of the matrix.

According to a second aspect, the present disclosure relates to a methodof making the core-shell ZSM catalyst, involving treating a ZSM-5zeolite with a silicalite gel including a silicating agent (e.g. SiO₂),a structure directing agent, a mineralizing agent, and water.

Preferably, the ZSM-5 zeolite is calcined at a temperature in the rangeof 500 to 650° C., preferably about 550° C. before being treated withthe silicating solution. The terms ZSM-zeolite and ZSM-5 core are usedinterchangeably herein.

In a preferred embodiment, the ZSM-5 zeolite is treated with thesilicalite gel at a temperature in the range of 150° C. to 250° C.,preferably about 200° C., and a pressure in the range of 2 to 20 bars,preferably 2 to 10 bars, more preferably 2 to 5 bars. The temperatureand the pressure may be provided via an autoclave. Accordingly, thesilicalite gel, which may be a suspension solution, is prepared first bymixing the structure directing agent, the mineralizing agent, and water.Next, a predetermined amount of the ZSM-5 zeolite is added to thesilicalite gel having the silicating agent, the structure directingagent, the mineralizing agent and water. Preferably, a ratio of thesilicating agent to the structure directing agent to the mineralizingagent to water is in the range of 0.5:0.05:0.1:15 to 1.5:0.1:2.0:25,more preferably about 1:0.08:1.6:20. A mineral acid, for examplehydrochloric acid, may be added to the silicalite gel to adjust a pH tobe within the range of 5-10, preferably 5-7. Further, the silicalite gelis sealed in the autoclave, which may preferably be made of stainlesssteel with a Teflon® liner, and the sealing is provided by metal endcaps and temperature resistant rubber gaskets. The ZSM-5 zeolite ispreferably treated with the silicalite gel to form a layer of silica onthe ZSM-5 zeolite. Treating the ZSM-5 zeolite with the silicalite gelpreferably refers to holding the ZSM-5 zeolite in the silicalite gel ata temperature in the range of 150° C. to 250° C., preferably about 200°C., and a pressure in the range of 2 to 20 bars, preferably 2 to bars,more preferably 2 to 5 bars, for at least 1 day, preferably at least 5days, more preferably at least 15 days

After the treatment, the ZSM-5 zeolite is filtered and the filtrate ispreferably washed with deionized water, followed by a drying at 80 SC,preferably 70° C., for at least 1 day, preferably about 2 days. Thedried ZSM-5 zeolite may further be calcined at a temperature in therange of 500 to 650° C., preferably about 550° C. to form the core-shellZSM catalyst having a silica shell with a thickness of 0.5 to 15 μm.

In a preferred embodiment, the core-shell ZSM catalyst, which hasalready been calcined, is treated one more time, with a substantiallysimilar treatment method to form another silica shell on the previouslyformed silica shell. Since the newly formed silica shell isindistinguishable from the former silica shell, and preferably nointermediate layer exists therebetween, the resulting catalyst after thesecond treatment may preferably be referred to as “the core-shell ZSMcatalyst” having a silica shell with a thickness of 0.5 to 50 μm,preferably 5 to 50 μm, more preferably 10 to 40 μm.

The method of making the core-shell ZSM catalyst may involve repeatingthe calcining and the treating-until the thickness of the silica shellis in the range of 0.5 to 50 μm, preferably 5 to 50 μm, more preferably10 to 40 μm.

Preferably, the core-shell ZSM catalyst is treated at least once, but nomore than three times. Accordingly, a core-shell ZSM catalyst that hasbeen treated once may have an acidity in the range of 0.05-0.15 mmol/g,preferably about 0.1 mmol/g, whereas this quantity reduces to less than0.01 mmol/g, when the core-shell ZSM catalyst is treated twice. In oneembodiment, a ratio of the acidity of the core-shell ZSM catalyst to theZSM-5 zeolite is no more than 0.2, preferably no more than 0.1, and noless than 0.01, preferably no less than 0.05, when the core-shell ZSMcatalyst is treated twice.

In addition, the size of zeolite crystals of the ZSM zeolite inaccordance with the method of the second aspect may be in the range of2-5 μm, preferably 2-4 μm, more preferably about 3.5 μm.

In one embodiment, a weight percent of the silica shell in thecore-shell ZSM catalyst, which is made in accordance with the secondaspect, is within the range of 4 to 75 wt %, preferably 30 to 75%, morepreferably 50 to 75 wt %, even more preferably about 65 wt %, with theweight percent being relative to the total weight of the core-shell ZSMcatalyst.

In one embodiment, the structure directing agent is a quaternaryammonium salt with a chemical formula N(C_(n)H_(3n))₄ ⁺X⁻, with X beinga halide ion or a hydroxide, and n being a positive integer between 1 to5. Exemplary halide ions include but are not limited to, fluoride,chloride, bromide, and iodide. Accordingly, exemplary structuredirecting agents include but are not limited to tetramethylammoniumhydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide,tetrabutylammonium hydroxide, tetrapentylammonium hydroxide,tetramethylammonium fluoride, tetraethylammonium fluoride,tetrapropylammonium fluoride, tetrabutylammonium fluoride,tetrapentylammonium fluoride, tetramethylammonium chloride,tetraethylammonium chloride, tetrapropylammonium chloride,tetrabutylammonium chloride, tetrapentylammonium chloride,tetramethylammonium bromide, tetraethylammonium bromide,tetrapropylammonium bromide, tetrabutylammonium bromide,tetrapentylammonium bromide, tetramethylammonium iodide,tetraethylammonium iodide, tetrapropylammonium iodide,tetrabutylammonium iodide, tetrapentylammonium iodide. In a preferredembodiment, the halide ion is bromide, and the structure directing agentis tetrapropylammonium bromide. In an alternative embodiment, thestructure directing agent is cetyltrimethylammonium chloride,cetyltrimethylammonium bromide, cetyltrimethylammonium iodide,cetyltriethylammonium bromide, cetyltriethylammonium chloride,cetyltriethylammonium iodide, or any combination thereof.

In one embodiment, the mineralizing agent is a fluoride salt. The term“fluoride salt” refers to a chemical compound consisting of a cation anda fluoride anion. Preferably the fluoride salt is ammonium fluoride(NH₄F) and/or ammonium bifluoride (NH₄HF₂).

In an alternative embodiment, sodium hydroxide may be utilized as themineralizing agent, as a replacement for the fluoride salt.Alternatively, sodium hydroxide may also be used in combination withammonium fluoride (NH₄F) and/or ammonium bifluoride (NH₄HF₂).

Preferably, the silicating agent is silica (SiO₂), even though othersilicating agents such as tetraethylorthosilicate (TEOS),tetramethylorthosilicate (TMOS), or polydimethylsiloxane (PDMS) may alsobe used individually or in combination with silica. Alternatively,sodium silicate, tetramethylammonium silicate, and/or sodiummetasilicate may be utilized in conjunction with the silicating agent.In one embodiment, a weight ratio of the ZSM-5 zeolite to the silicatingagent is in the range of 1:2, preferably 1:10, more preferably 1:25.

According to a third aspect, the present disclosure relates to a methodof making the core-shell ZSM catalyst, involving treating a ZSM-5zeolite with a silicating solution including an organic solvent and asilicating agent.

Preferably, the ZSM-5 zeolite is calcined at a temperature in the rangeof 500 to 650° C., preferably about 550° C. before being treated withthe silicating solution.

In a preferred embodiment, the ZSM-5 zeolite is treated with thesilicating solution at the boiling point of the organic solvent, whichmay be in the range of 50-120° C., preferably about 70° C., and atatmospheric pressure.

Accordingly, a predetermined amount of the ZSM-5 zeolite is mixed withthe organic solvent. Exemplary organic solvents include, but are notlimited to acetone, acetonitrile, chloroform, diethyl ether,dimethylacetamide, dimethylformamide, dimcthylsulfoxide,isopropylalcohol, dioxane, ethanol, ethyl acetate, hexane, methanol,n-methylpyrrolidinone, pyridine, tetrahydrofuran, toluene, or acombination thereof. Preferably, the organic solvent is hexane with aboiling point of about 70° C.

The mixture of the ZSM-5 zeolite and the organic solvent is stirred,while concurrently heated until the temperature of the mixture reachesto the boiling point of the organic solvent. At this point, the organicsolvent refluxes to the mixture, and a predetermined amount of thesilicating agent (preferably about 4 wt %, relative to the total weightof the silicating solution) is added to the mixture, preferably in adropwise manner, to form the silicating solution. The silicatingsolution is further stirred for at least 2 hours, preferably at least 5hours, while the organic solvent is continuously refluxed at the boilingpoint of the organic solvent, whereby silica may start to deposit on theZSM-5 zeolite, and the core-shell ZSM catalyst is formed. After that,the organic solvent may be removed from the silicating solution viavacuum evaporation, leaving behind the core-shell ZSM catalyst. Afterthe treatment, the core-shell ZSM catalyst is filtered and the filtrateis preferably washed with deionized water, followed by a drying at 80°C., preferably 100° C., for at least 1 day, preferably about 2 days. Thedried core-shell ZSM catalyst may further be calcined at a temperaturein the range of 500 to 650° C., preferably about 550° C.

The core-shell ZSM catalyst produced via the method of the third aspectmay have a silica shell thickness in the range of 0.5 to 5 m. However,the core-shell ZSM catalyst, which has already been calcined, may betreated few more times, with a substantially similar treatment method toform one or more silica shells on the previously formed silica shell.Since the newly formed silica shell is indistinguishable from the formersilica shell, and preferably no intermediate layer exists therebetween,the resulting catalyst after the last treatment may preferably bereferred to as “the core-shell ZSM catalyst” having a silica shell witha thickness of 0.5 to 30 μm, preferably 5 to 30 μm, more preferably 5 to25 μm.

Preferably, the core-shell ZSM catalyst is treated at least once, but nomore than six times. Accordingly, a core-shell ZSM catalyst that hasbeen treated once may have an acidity in the range of 0.1-1.5 mmol/g,preferably about 0.1-0.75 mmol/g, whereas this quantity reduces to lessthan 0.4 mmol/g, preferably less than 0.3 mmol/g, more preferably lessthan 0.2 mmol/g, when the core-shell ZSM catalyst is treated at leastthree time, preferably six times. In one embodiment, a ratio of theacidity of the core-shell ZSM catalyst to the ZSM-5 zeolite is in therange of 0.45 to 0.65, preferably 0.5 to 0.65, when the core-shell ZSMcatalyst is treated six times.

In addition, the size of zeolite crystals of the ZSM zeolite inaccordance with the method of the third aspect may be less than 2 μm,preferably less than 1 μm.

In one embodiment, a weight percent of the silica shell in thecore-shell ZSM catalyst, which is made in accordance with the secondaspect, is within the range of 4 wt % to 30 wt %, preferably 15 wt % to30 wt %, even more preferably 15 wt % to 25 wt %, with the weightpercent being relative to the total weight of the core-shell ZSMcatalyst.

In a preferred embodiment, the silicating agent is silica (SiO₂), eventhough other silicating agents such as tetraethylorthosilicate (TEOS),tetramethylorthosilicate (TMOS), or polydimethylsiloxane (PDMS) may alsobe used individually or in combination with silica. Alternatively,sodium silicate, tetramethylammonium silicate, and/or sodiummetasilicate may be utilized in conjunction with the silicating agent.

The examples below are intended to further illustrate protocols for themethod of producing propylene and ethylene from a butene-containinghydrocarbon stream, and the methods of making the core-shell ZSMcatalyst, and are not intended to limit the scope of the claims.

EXAMPLE 1

The objective of the present disclosure is to modify ZSM-5 catalyst inorder to improve ethylene and propylene yield from the catalyticcracking of 1-butene. The modified ZSM-5 catalyst presented in thisdisclosure may be used in stand-alone units dedicated for the crackingof 1-butene or butene mixtures, which are produced from FCC crackers orethylene crackers. The modified procedure involves a surfacemodification technique using silica by chemical liquid deposition methodor core-shell silicalite composite synthesis. The present inventionproduces a higher yield of ethylene and propylene over prior catalystswith different P/E ratio. Additional objects, embodiments, and detailsof this invention can be obtained from the description of the invention.

EXAMPLE 2

A method for surface modification of ZSM-5 catalysts in the cracking of1-butene to light olefins is provided. The MFI crystal structure withintersecting 10 membered-ring pore channels having a SiO₂/Al₂O₃ ratio(23, 80, 280 to 1500) were used for surface modification using chemicalliquid deposition (CLD) method. In each cycle, 4 wt % of silica wasdeposited using tetraethylorthosilicate as the silica source. The numberof silica deposition cycle on each material is described in Table 1. Ineach cycle of silica deposition, the acidity of the material wasdecreased as shown in Table 2.

TABLE 1 The catalysts used for catalytic cracking of 1-butene. CatalystCode Description A CBV 2314 from Zeolyst International calcined in airat 550° C. for 3 hrs. SiO₂/Al₂O₃ (mol/mol) = 23. A1 3 times silicadeposition using CLD method on catalyst A A2 6 times silica depositionusing CLD method on catalyst A B CBV 8014 from Zeolyst Internationalcalcined in air at 550° C. for 3 hrs. SiO₂/Al₂O₃ (mol/mol) = 80. B1 3times silica deposition using CLD method on catalyst B B2 6 times silicadeposition using CLD method on catalyst B C CBV 28014 from ZeolystInternational calcined in air at 550° C. for 3 hrs. SiO₂/Al₂O₃ (mol/mol)= 280. C1 3 times silica deposition using CLD method on catalyst C C2 6times silica deposition using CLD method on catalyst C C3 Silicadeposition by core-shell synthesis using catalyst C C4 Silica depositionby core-shell synthesis using catalyst C3 D 890HOA from TOSOH Japan,calcined in air at 550° C. for 3 hrs. SiO₂/Al₂O₃ (mol/mol) = 1500 D1 Onetime silica deposition using CLD method on catalyst D

TABLE 2 Results of temperature programmed desorption of ammonia (TPD)using parent and modified ZSM-5 catalysts. NH₃-TPD (mmol/g)Catalyst >300° C. 300-550° C. Total Acidity A 0.46 0.29 0.75 A1 0.310.16 0.47 A2 0.28 0.14 0.42 B 0.27 0.13 0.40 B1 0.21 0.11 0.33 B2 0.120.09 0.21 C 0.08 0.07 0.15 C1 0.07 0.05 0.12 C2 0.06 0.04 0.10 C3  nd*nd nd C4 nd nd nd D 0.05 0.01 0.06 D1 nd nd nd *nd = not detectable

EXAMPLE 3

Core-shell silicalite composite was synthesized using SiO₂/Al₂O₃ ratio280. The parent material was mixed with gel composition of 1 SiO₂: 0.08TPABr: 0.1-2.0 NH₄F: 20H₂O. The ratio of parent material and silica inthe gel was about 1:2 (catalyst C3). Using sample gel composition,catalyst C3 was used as parent material for the synthesis of catalystC4. The catalyst C3 and C4 show very weak acidic sites which well belowthe detection limit of TPD of ammonia analysis (Table 2). Weak acidsites act as an efficient catalyst for the formation of ethylene andpropylene whereas strong acidic sites promote the formation of alkanesand aromatics. The acid sites on core-shell silicalite catalysts(Catalyst C3 and C4) are so weak and light olefins can easily beprotonated to form carbocations. These weak acid sites may be able toform carbocations to initiate the acid-catalyzed reactions particularlyat elevated temperatures (because the strength of acid sites increaseswith temperature). The absence of strong acid sites on the core-shellsilicalite composite prevents the olefins from further conversion toalkanes and aromatics via hydrogen transfer reactions.

EXAMPLE 4

The SEM images of catalyst C, 6 times silica deposited on catalyst C(C2), core-shell silicalite composite of the catalysts C3 and C4 wasshown in FIG. 1. There was no change in morphology and crystal size ofthe catalysts C and C2. However, the crystal size of the catalysts C3and C4 found to be about 2-3 m larger than that of the parent zeolite(Catalyst C).

EXAMPLE 5

The operating conditions of the 1-butene cracking reaction comprisetemperatures between about 450° C. to about 700° C. A preferredtemperature for operating the process is to be within the range fromabout 500° C. to about 650° C. with a more preferred operatingtemperature of about 540° C. to about 600° C. The reaction processoperation conditions further include hydrocarbon partial pressuresbetween about 35 kPa (5 psia) to about 345 kPa (50 psia). The catalyticprocess of 1-butene cracking can be conducted in a continuouscirculating fluidized-bed reactor or in a fixed-bed reactor. The gashourly space velocity (GHSV) based on 1-butene feed can be within therange 800 hr⁻¹ to 10000 hr⁻¹.

EXAMPLE 6

The product yields of 1-butene cracking over ZSM-5 catalysts comprisevarious hydrocarbon groups depending on the expected pathway to formdifferent products. These groups are; 1-butene (reactant), cis- andtrans-2-butene (double bond isomerization products), ethylene, propylene(target product), other olefins (pentenes, and hexenes) and n-buteneisomers (skeletal isomerization and cracking products), alkanes(hydrogen transfer products), aromatics (benzene, toluene, xylenes andethylbenzene; hydrogen transfer products), and C₈₊ hydrocarbons(aromatics other than benzene, toluene, xylenes, and ethylbenzene,alkanes and olefins). Using modified ZSM-5 as the cracking catalyst for1-butene may improve light olefin yields formation due to minimum sidereactions such as isomerization and hydrogen transfer reactions, whichlead to formation of C₈₊ alkanes and aromatics.

EXAMPLE 7—Silica Deposition Using Chemical Liquid Deposition Method

Silica coating on ZSM-5 zeolites with a different SiO₂/Al₂O₃ ratio of23, 80 and 280 was carried out using chemical liquid deposition (CLD)method. Before silica coating zeolites material were calcined at 550° C.for 3 h with a heating rate of 5° C./min. In a typical coatingprocedure, 10 grams of parent zeolite was suspended in 100 ml ofn-hexane solvent and the mixture was heated to the boiling point ofn-hexane at 70° C., wherein n-hexane is refluxed. After 30 minutes ofstirring and refluxing, a TEOS solution that corresponds to a loading ofabout 4 wt % SiO₂ was added and silylation was continued for 2 h at 70°C. with stirring and refluxing. Excess n-hexane was removed byevacuation. Finally, the sample was dried at 100° C. for 24 h andcalcined at 550° C. for 4 h, with a heating rate of 5° C./min. Aftereach TEOS deposition 2 g of catalysts were taken from a batch andsubjected to physicochemical characterization and catalyst activitytest. Silylation treatment was carried out up to six times using thesame procedure.

EXAMPLE 8—Synthesis of Core-Shell Composite ZSM-5(280) @Silicalite-1(1×) Catalysts (Catalyst C3)

HZSM-5(280) @silicalite-1 core-shell composite was prepared using tetrapropyl ammonium bromide as structure directing agent and ammoniumfluoride as a mineralizer. In a typical synthesis procedure, 6 g ofZSM-5 (280) zeolite was mixed well with silicalite-1 gel prepared usingthe following molar composition of the gel 1 SiO₂: 0.08 TPABr: 1.6 NH₄F:20H₂O. This was subjected to hydrothermal process at 200° C. for 2 days.The sample was then washed with deionized water, filtered, dried andcalcined at 550° C. for 6 h.

EXAMPLE 9—Core-Shell Synthesis of ZSM-5(280)@ Silicalite-1(2×) Catalysts(Catalyst C4)

6 g of ZSM-5(280)@ silicalite-1 was mixed well with silicalite-1 gelprepared using the following molar composition of the gel 1 SiO₂: 0.08TPABr: 1.6 NH₄F: 20H₂O. This was subjected to hydrothermal process at200° C. for 2 days. The sample was then washed with deionized water,filtered, dried and calcined at 550° C. for 6 h.

EXAMPLE 10—Catalytic Performance of Modified ZSM-5 Catalysts

The catalytic performance of the modified ZSM-5 catalysts was evaluatedin a fixed-bed packed with 2 ml of the catalyst with a particle size of0.5-1.0 mm diameter. The catalyst sample was pretreated in a nitrogenstream at 550° C. for 1 h and then a mixture of the 1-butene andnitrogen (5 ml/min and 25 ml/min, respectively) (GHSV=900 h⁻¹) waspassed through the catalyst bed at 550° C. The products were analyzed byan on-line gas chromatographer equipped with a GS-Gaspro column and aflame ionization detector (FID).

EXAMPLE 11

The results of catalytic performance of 1-butene cracking using parentzeolite as well as modified zeolite catalysts are presented in FIG. 2and Table 3. Catalyst C4 revealed the highest propylene and ethyleneyield (55%), with a P/E ratio of approximately 3, when compared to othercatalysts. After surface modification by silica deposition usingchemical liquid deposition method, as in catalysts A, B, and C, thepropylene and ethylene yield was improved giving a P/E ratio of lessthan 2. Catalysts D and DI revealed less activity with a slight increasein the P/E ratio, when compared to the catalyst C4. Kinetics of theconversion reactions of butene to propylene and ethylene over thecatalyst C4 and the catalyst D is shown in FIG. 3, wherein concentrationof each of the reactants and the products are plotted versus time in a50-hour time frame. The results as shown in FIG. 3 reveal that thecatalyst D deactivates faster than the catalyst C4. More importantly,the yield of propylene and ethylene formation remains unchanged over aperiod of 50 hours, when catalyst C4 was used, whereas the yield wasconsiderably reduced over the period of 50 hours, when catalyst D wasused. The results further represent a greater stability of thecore-shell ZSM catalyst (i.e. the catalyst C4) than that of the catalystD.

TABLE 3 Products distribution (C-wt %) in cracking of 1-butene usingdifferent catalysts at 1 bar, 550° C. (TOS = 1 hr, GHSV = 900 h⁻¹). A A1A2 B B1 B2 C C1 C2 C3 C4 D D1 1-C₄═ Conversion 100.00 100 98.34 100.00100.00 97.90 98.30 97.84 97.81 96.92 95.29 91.57 92.61 Productdistribution (C-wt %) 1-butene 0.00 0.00 1.66 0.00 0.00 2.10 1.70 2.162.19 3.08 4.71 8.43 7.39 2-butene 0.00 1.06 3.48 0.00 1.02 4.35 3.545.50 4.53 6.36 9.73 16.49 14.71 Isobutylene 0.00 2.27 4.03 1.56 2.055.13 3.85 5.72 4.94 7.06 11.13 19.77 17.28 C₂═ 1.70 12.52 15.93 12.6612.29 17.18 18.58 18.27 20.07 18.89 13.82 8.84 9.62 C₃═ 0.50 13.30 20.5811.49 13.11 24.65 21.88 28.14 26.37 32.87 41.44 31.62 35.11 C₄═ 0.003.33 9.16 1.56 3.07 11.58 9.09 13.38 11.66 16.50 25.57 44.69 39.38 C₅═0.00 0.00 0.82 0.00 0.00 1.05 0.82 1.31 1.08 3.23 5.45 6.88 6.64 C₆═0.00 11.52 5.14 8.92 8.61 4.72 4.61 3.96 4.09 2.46 0.98 0.00 0.86Alkanes (C₁-C₆) 14.80 32.91 27.12 35.60 32.60 22.89 22.11 17.23 17.8614.39 7.08 4.59 4.43 Aromatics (BTEX) 50.50 18.75 12.96 19.67 22.7111.35 12.46 11.15 12.23 6.33 0.00 0.00 0.00 C₈₊ 32.60 7.68 8.29 10.107.60 6.58 10.44 6.56 6.64 5.33 5.68 3.38 3.95 C₂═ + C₃═ 2.20 25.82 36.5124.15 25.41 41.83 40.46 46.40 46.44 51.76 55.26 40.46 44.73 P/E ratio0.29 1.06 1.29 0.91 1.07 1.43 1.18 1.54 1.31 1.74 3.00 3.58 3.65

The invention claimed is:
 1. A method of cracking a butene-containing hydrocarbon stream to produce a mixture of propylene and ethylene, comprising: contacting the butene-containing hydrocarbon stream with a core-shell ZSM catalyst in a fixed-bed reactor to form a product stream comprising propylene and ethylene, wherein at least 50 wt % of the butene-containing hydrocarbon stream is butene, and wherein the core-shell ZSM catalyst comprises: a ZSM-5 core, and a silica shell having a thickness in the range of 0.5 to 50 μm, which covers at least a portion of a surface of the ZSM-5 core.
 2. The method of claim 1, wherein the silica shell has a thickness in the range of 0.5 to 30 μm.
 3. The method of claim 1, wherein the core-shell ZSM catalyst is dispersed in a silica and/or an alumina binder.
 4. The method of claim 1, wherein a weight percent of the silica shell in the core-shell ZSM catalyst is within the range of 4 to 75 wt %, with the weight percent being relative to the total weight of the core-shell ZSM catalyst.
 5. The method of claim 1, wherein the core-shell ZSM catalyst has an acidity of less than 0.1 mmol/g.
 6. The method of claim 1, wherein at least 50 wt % of the product stream is propylene and ethylene.
 7. The method of claim 1, wherein a propylene-to-ethylene weight ratio of the product stream is within the range of 0.2 to
 4. 8. The method of claim 1, further comprising: treating the core-shell ZSM catalyst with nitrogen at a temperature in the range of 400 to 700° C. prior to the contacting.
 9. The method of claim 1, further comprising: mixing the butene-containing hydrocarbon stream with nitrogen to form a gaseous mixture prior to the contacting, wherein a partial pressure of the butene-containing hydrocarbon stream in the gaseous mixture is within the range of 5 to 50 psi.
 10. The method of claim 1, wherein the butene-containing hydrocarbon stream is contacted with the core-shell ZSM catalyst at a temperature in the range of 400 to 700° C. and a space velocity in the range of 800 to 10,000 h⁻¹. 